ESRL/PSD Seminar Series

Abstract

Deficits in precipitation are the primary cause of drought development at monthly and longer time scales, but other climatic factors such as temperature, solar radiation, wind, and humidity play a central role in rapid onset droughts, recently termed "flash drought." The evolution of flash drought often begins with ample soil moisture during the spring and early summer, leading to high, energy-limited actual evapotranspiration (Ea) rates. Although precipitation may remain near normal, this situation nonetheless leads to rapid decreases in soil moisture, forcing the land-surface energy balance from energy-limited to water-limited conditions. During such transitions, feedbacks between the land and the near-surface boundary layer accelerate and intensify the evaporative demand (Eo) though increases in sensible heat, and in the buoyancy, turbulence, and vapor pressure deficit of the over-passing air. These complex feedbacks and the resulting evolution of a flash drought are not fully captured with precipitation and temperature-based indicators alone. This observation motivates the development of a physically based metric that is sensitive to changes in solar radiation, temperature, humidity, and wind through the use of the ASCE Standardized Penman-Monteith equation to quantify Eo anomalies, termed the Evaporative Demand Drought Index (EDDI). EDDI is compared against precipitation and soil moisture anomalies, a satellite based Evaporative Stress Index, and the United States Drought Monitor. Results suggest that the EDDI could prove to be a very useful, and easy-to-implement, operational early warning drought monitoring tool, as well as a long-term hydrologic drought metric. Providing reliable seasonal drought forecasts continues to pose a major challenge for scientists, end-users, and the water resources community. Precipitation is the most commonly used variable to assess future drought and water supply outlooks. However, studies have shown that precipitation forecasts at seasonal lead times are highly uncertain, and skill drops off dramatically past one month. Forecasts of accumulated Eo anomalies could provide additional information and early warning to water managers and the agricultural community. My current research explores using Eo anomalies as a seasonal drought forecasting tool over the United States. A 28-year (1982-2009) reforecast of the Climate Forecast System version 2 (CFSv2) and the CFS reanalysis is used to establish the climatology and historical skill of CFSv2 in predicting 2-m temperature (maximum and minimum), 2-m specific humidity, 10-m wind speed, downwelling shortwave radiation at the surface, and ASCE Standardized Reference ET over the CONUS. Some initial results and future direction of the project will be shared in this talk.